1. Field of the Invention
The present invention concerns in general terms an antenna device, notably for a telecommunication station, and more particularly an antenna device comprising a periodic arrangement of radiating elements.
2. Description of Related Art
Recent developments in antenna systems for mobile telephony have revealed an increasing interest in so-called xe2x80x9cintelligentxe2x80x9d antennae. Such an antenna consists of a array of elementary antennae with a pitch less than or equal to the half-wavelength of the transmission frequency, which can be used both for transmission and for reception. Depending on circumstances, the input or output signals of the elementary antennae are offset in phase and weighted so as to obtain the required radiation diagram. Thus, for example, an intelligent antenna equipping a base station can form a beam pointing in the direction of a mobile terminal and/or eliminate interference coming from a given direction.
There are also known mobile telephony systems referred to as spatial diversity systems for combating the fading in the signal due to propagation on multiple paths. These systems use a plurality of antennae, spaced apart typically by four to ten times the wavelength of the transmission frequency and taking advantage of the fact that the signals received by sufficiently distant antennae are decorrelated. Thus signals which have propagated on several paths as far as a first antenna (the first diversity branch) giving rise to destructive interference can on the other hand give rise to constructive interference at another antenna (the second diversity branch). The reception diversity is then exploited, for example, in that, at a given moment, the diversity branch giving the best signal to noise ratio (Selective Combining) is selected or the different branches are summed after they have been weighted by a gain equal to the conjugate complex of the complex attenuation coefficient on the branch under consideration (Maximal Ratio Combining).
The antenna arrays as mentioned above do not deal well with spatial diversity functioning since the signals received by two consecutive antennae are generally not sufficiently decorrelated. It can then be thought of increasing the pitch of the array, as proposed in the article by H. Yoshinaga et al. entitled xe2x80x9cPerformance of adaptive array antenna with widely spaced antenna elementsxe2x80x9d which appeared in the Proceedings of VTC ""99, pages 72-76. However, increasing the pitch inevitably introduces array lobes in the radiation diagram, which impairs the spatial selectivity of the system.
It is also known that mobile telecommunications systems use different transmission frequencies, typically 900 MHz and 1800 MHz for GSM systems, 2 GHz for future UMTS systems and even higher frequencies, probably in the 20-30 GHz band, for satellite mobile telephony. The antenna arrays being, as has been seen, designed for a given frequency, the intelligent antennae used for one generation in mobile telephony will not operate or will operate badly with the following generation. The operator must then, with each generation, bear the considerable cost of new equipment.
The aim of the invention is to propose an intelligent antenna which does not have the aforementioned drawbacks, namely allowing both the shaping of a beam and diversity reception and being able to adapt easily to a new mobile telephony.
To this end, the antenna device according to the invention comprises a plurality of radiating elements disposed in a periodic arrangement having at least one spatial periodicity, characterised in that it has antenna configuration means adapted to vary the value of the said spatial periodicity according to the transmission conditions.
According to a first characteristic of the invention, the antenna device comprises a beam shaper adapted to form a beam in at least a first direction using input signals to and/or output signals from radiating elements.
According to a second characteristic of the invention, the antenna device comprises at least one beam shaper adapted to reject an interfering signal in at least a second direction using the output signals from the radiating elements.
According to a third characteristic of the invention, the antenna device comprises a receiver or transmitter adapted to receive: or transmit in spatial diversity.
Another embodiment of the invention comprises a mixed system able to function either as a beam shaper or as a receiver in spatial diversity, the configuration means fixing the pitch of the array at a value less than or equal to a half-wavelength when the mixed system is functioning as a beam shaper and at a value substantially greater than the wavelength when it is functioning as a spatial diversity receiver.
Advantageously, the configuration means are adapted to place the antenna in an intermediate configuration without antenna processing during the phase of variation in the spatial periodicity.
Advantageously again, the configuration means comprise hysteresis or timing means able to eliminate unwanted changes in the said spatial periodicity.
According to a first variant of the invention, the configuration means comprise at least one rail in which the supports for the radiating elements can slide.
According to a second variant of the invention the radiating elements consist of a plurality of elementary antennae. The configuration means comprise a plurality of units adapted to weight and sum a set of output signals from adjacent elementary antennae, a switch directing certain output signals from the said units to the inputs of at least one beam shaper, the spatial periodicity of the radiating elements being modified by selecting the output signals from elementary antennae and the output signals from these units.
Advantageously, the transmission conditions are one or more characteristics of the transmission amongst the bit error rate, the packet error rate, the ratio between the power of the signal to noise plus interference, the quality of service and the power consumed by the transmitter responsible for the transmission.
Finally, the antenna device according to the invention can be integrated into a mobile terminal or a base station.